The metadata property shows user-defined metadata associated
with this server.
Setting and updating metadata shows how you can create or delete metadata.

Some metadata are set by Rackspace and not available to be changed by
the user. Generally these metadata are used by automation and
orchestration systems needed to correctly build and maintain your server.

The metadata that are attached to your server are also stored if you
create a snapshot or image from the server, and are restored when a server is
built from that image. The key names are prefixed with instance_.